The planet in question is this one. Approximately three quarters the size and half the gravity of Earth, an atmosphere 25% as dense as the Terran one which mainly consists of Nitrogen with small percentages of Oxygen and Carbon Dioxide. Orbiting a Proxima Centauri, a red dwarf, at the outer rim of the habitable zone, the planet is relatively cold, with average surface temperatures ranging from -80 to -30°C. Volcanic activity is non-existent or minimal at most. The sky takes on a dark greyish-brown color, becoming reddish at sunrise and -set and during dusk as well as around the star.

The planet rotates within 27 hour; its orbital period is approximately 20 days. The orbit is rougly circular; the axial tilt of the planet is relatively high (34°).

Seas of liquid ammonia cover approximately half of the planet where it is not frozen. Color of the sea varies from deep blue to golden bronze, varying by the amount of metals dissolved in the ammonia. The colors render the structure of oceanic currents.

I have heard that ammonia quickly and easily dissolves soil, so I imagine that the terrain, especially the coastlines, would be very ragged. I imagine that ammonia would only boil or evaporate in few places and quickly rain down, which means that most rivers would be periodic and located close to coasts, albeit leaving distinct marks on the terrain.

The topmost layers of the rock or soil will probably contain nitrogen compounds, their concentration rising as you get closer to the nearest coast. This will be interesting especially if those compounds have distinct colors.

Terrain close to the coasts will be very ragged, full of small ammonia lakes or ammonia ice patches, canyons and valleys made by rivers while further away from the sea, it will mainly consist of plains and rolling hills with occasional meteorite craters.

Because ammonia does not have a density anomaly like water, I imagine that the further north or south you travel, the higher up the sea will be frozen. While only the deepest areas of the equatorial lakes and oceans will be frozen, the poles will be covered in vast and very flat ammonia sea plains and glaciers.

My questions are:

Is my description plausible?

If not, why? What must be changed?

Can you add anything? Weather system? Climate? How would ammonia clouds look?

3 Answers
3

Your description seems plausible to me. I might add a few things which may or may not have been considered.

Volcanic Activity

Given that your planet has no volcanic activity, its core has cooled, leaving no magnetic field and leaving the atmosphere vulnerable to being stripped away by solar/interstellar winds. These winds will pick off the least dense elements of your atmosphere, so nitrogen will be a prime target.

Thin Atmosphere

With a thin atmosphere, you'd expect less weathering, meaning high mountains. With no geological activity, however, mountains have more time to be worn down. The terrain of your planet depends on how long ago your planet stopped being geologically active and how windy it is. I detail below that your planet is windy, so I expect the rolling hills and plains that you mentioned, even for a planet with half the gravity.

A thin atmosphere with mostly nitrogen also means no greenhouse effect, so your surface temperature is close to the effective temperature of the planet. The actual numbers are important, since I assume you want the ammonia freezing point to be somewhere within your temperature range: https://en.wikipedia.org/wiki/Effective_temperature

Axial Tilt

Your planet will experience more dramatic seasonal changes due to its high axial tilt. This puts your tropic latitudes at +/-34 degrees and your arcitcs' at +/-56 degrees. What this means for your weather:

Bigger surface temperature range

Lakes can actually freeze near the equator, since a summer at the equator will be colder than a summer at the arctic.

Since your seasons are 5 days long each, you'll have a dramatic temperature differential, which is the source of most weather events on Earth. Very windy planet.

The emission spectrum of a red dwarf would be much different from that of our Sun. When you say

The sky takes on a dark greyish-brown color, becoming reddish at sunrise

and

Color of the sea varies from deep blue to golden bronze

do you refer to "our" colors, or to the local equivalent in the red-dwarf spectrum? Expecially blue would be hard to see, in a reddish light.

Furthermore, you state gravity is half the one on Earth, but yet you want to have a denser atmospehere. How? The density of the atmosphere is dependent on the gravity, in other word the stronger the planet can "hug" its gaseous blankets with gravity, the denser the atmosphere will be.
Lower gravity mean less dens atmosphere.

Last point, with lower pressure (due to lower gravity) how are you going to keep ammonia liquid? I have the feeling the temperatures and pressures you imply in your description gives only gaseous ammonia.

There may be some fundamental problems with this planet that supercede concerns about its appearance.

A terrestrial planet with half Earth's gravity and a quarter its atmospheric density would have a very difficult time retaining the hydrogen necessary for ammonia (NH$_3$). Even in orbit of a weak red dwarf, your planet would be subject to hydrodynamic escape (consider Titan, 9.54 AU out, also experiences this) from stellar wind (Proxima Centauri has about 20% Sol's strength). The lack of significant volcanism also suggests an inactive core and therefore a weak magnetosphere, further exposing the hydrogen. (As hydrogen escapes the remaining nitrogen and oxygen will form nitric oxide (NO) and nitrogen dioxide (NO$_2$) and nitrous oxide (N$_2$O).)

As L.Dutch points out, the pressure and temperature range you've given has your ammonia oceans flash-boiling, reliquifying, and then freezing. Consult this phase diagram for ammonia, locating your temperature range and pressure near the bottom left. Raising atmospheric pressure to Earth-standard would prevent this.

This planet would also have a tougher time retaining heat to prevent the anti-stellar pole from completely freezing over and dooming the rest of the world to a Snowball situation – I doubt a 20-day year is enough time for winter ice to thaw before the next winter begins – and much larger polar regions means a larger share of the planet is going to freeze in darkness each season.

As for its appearance, unless there is life there won't be anything we'd recognize as soil, as that is a mixture of minerals and organic matter, but you would find sandy beaches and rocky shores as on Earth. Liquid ammonia's greater ability to dissolve minerals would make for steeper river valleys and estuaries that penetrate much deeper in land.